I'll get straight to the point. This was a difficult read, for two reasons:

It's long.

It's filled with errors.

You understand there's a point where you can't shorten this without cutting out story. However, you can get as close to that point as possible, by carefully choosing your words and crafting them. That, or you can compel the reader to endure through sheer force of narrative (which I believe any good writer can do).

What will repel your reader, however, is SPAG (Spelling, Punctuation, and Grammar). Each error is a bump in the road – which, in this case, is a very long road. Help make the case to read this by smoothing that out.
I've been told to recommend Grammarly to you. I haven't tried it myself, but having something to check for mistakes would help your writing immensely. Regardless of whether you use this, however:Clean this up.

This section inundates the reader with the details of the containment cell. The Foundation doesn't need to know that "a bed, a table, four chairs, toilets, a shower and a small shelf with board and card games" should be provided; they can figure out what basic furniture SCP-XXXX needs and shouldn't have as they go. Most of this description could be summarized under the specification "standard humanoid containment cell," with mention of the modifications made.

Speaking of these modifications: Why is this a corridor with two doors? And why is there a creation/destruction area attached to his cell if this is, effectively, the testing room? These left me confused.

The requirements keeping identity hidden from SCP-XXXX hooked me, though. Polishing these up and making these the focal point will strengthen your containment procedures and draw the reader in.

The first paragraph tries too hard to humanize this scip. I know your intent is to do so, but keep in mind that the Foundation would not include a physical description, education, and employment unless it pertained to its anomalous quality or containment. In this vein, mentioning the gain of weight, though it foreshadows that development, doesn't make sense; it would be better for the Foundation to just take the average. A lot of this can introduced in later sections (for example, the circumstances of acquisition; or the experiment logs, should you expand on them).

Your description of SCP-XXXX-1 and their effects (or lack thereof) is thorough. It can be superficially improved with formatting and phrasing (and SPAG, of course); but it's clear you've grasped the ideas behind these.

To answer your question about the use of more scientific terms: I'd recommend against it, for now at least. The description would benefit more from changes in the wording than the addition of more words.

I, unfortunately, see what you feel: This addendum has been rushed. The discovery log should read much like an action report in clinical tone, because this forms the preliminary basis for research and policy. Neither the verbatim rant of the informant nor the nature of their work relationship matter to the higher-ups; the details of the recovery operation matter more.
To illustrate my point – stuff like this needs to go:

as the co-worker said that "After the guy was done destroying my car with his fucking car, he removed his mask. And he had my face!", and pointed the deployed Tactical Response Team toward SCP-'s apartment, as the co-worker had, the morning of the infraction, blown cigarette smoke in SCP-'s face, who told him multiple times beforehand to "not smoke in the office".

The choice to deploy Agent Damian Sabio in this way was thought-provoking, because it showed the Foundation is willing to take the more humane route when available. His use of his wife's and daughter's pictures were cringe-worthy, however, not only because it came off as blatantly manipulative but because it exposes his family to a scip of which they know nothing.

Also, the ending was abrupt and confusing, in part because it's inconsistent with the description of the powers of SCP-XXXX (he has complete control). Perhaps SCP-XXXX panicked, and SCP-XXXX-1 interpreted this as necessitating lethal force; and the reason the former didn't command the latter to stop was because they were caught up in the moment.

I would tinker with these interactions, but otherwise keep the encounter and the ending much the same.

As these are experimentation logs, I feel you should present a table with the test subject, testing materials, hypothesis, and result. It would help to contextualize the following communication, and provide the contrast between the unfeeling bureaucracy of the Foundation and the emotional aspect of human dialogue.

You've correctly formatted the actions lines (the lines describing what a character does, as opposed to what they say). Action lines, as used in a scientific report, should be objective; and you do a good job at sticking to this. Don't say "they look __," especially when the person you're describing has a mask on. Case in point:

Doctor Stephanie Rom:<visibly annoyed by SCP-'s behaviour>

Here, you'd do better describing the tone of her voice without the emotion.
Action lines provide the opportunity to do what the writer's adage instructs us to do: show, not tell. As long as you abide by this, you can accomplish your goal of bringing the emotions to the forefront while keeping in good faith with the SCP style.

I get what you're trying to do here: show someone in the Foundation cares, someone's looking out for SCP-XXXX. What makes or breaks this is the contrast between his personal writing and the clinical tone of the other assessments, which is not distinctive enough for the former. And then, irrespective of this contrast, this detracts from one of the strongest dualities in your favor: the humanity of the dialogue against the coldness of the system.

The content of this assessment is worthwhile, but the style of its composition is dubious. I would make this look a little more professional, at least until you've nailed the rest of your writing.

First of all: You may have noticed I excluded the name "Selena Moore" from the title of this collapsible. There are three reasons for this:

This is the Site Director; her name is confidential. And unnecessary to provide: Everyone knows who she is, or at least how to contact her.

I sense you want this to be the final message from the machine that is the Foundation. Attaching a human name to it takes away from this.

Selena Moore is a minor character. Naming her siphons strength from the dynamic you've established between the named/titular characters: SCP-XXXX, Stephanie Rom, and Damian Sabio.

I'm going to be blunt: This message was cheesy. The first sentence:

I want to remind our personnel what the C in SCP means: Contain.

Sounds more like something from the mouth of a kindergarten teacher than from that of a site director.

It doesn't help that she tries to remind the personnel why SCP-XXXX is dangerous – "he's not in control," "he killed a guy," "he could do bad stuff", and "he poses a big threat." And then she wants to give the personnel (whom she refers to as 'you?') hope for his release? Finally, the mention that this kid could have been a Keter (when there was never a reason for him to be), and that he could be Thaumiel (when this was never considered in the article), feels tacked on.

You've raised the possibility of using SCP-XXXX-1 instances like D-Class, which is an important and intriguing idea; however, it appears lackluster attached to the last addendum. This is an idea that should have its own addendum, or it shouldn't appear at all.

I'd tell you to scrap this altogether, but there's something here you can salvage. I know what you want this message to be: A light at the end of the tunnel. You've gotta clean this up – the purpose and the choice of words – to make this belong. I hate to break it to you, but this feels more like LoLFoundation than resolution. And that must change if this is to stay.

I would talk to you about the concept of the scip.about how something that clones people combines tropes of doppelgängers and copy machines in an unoriginal way, but how SCP-XXXX needs to know their name and face like he's Light Yagami is kind of a cool twist

But the concept isn't the point. This is meant to capture the experience of being a sapient, humanoid scip. So I'll focus on the aspect of writing that will drive this home: Character.

It sounds weird at first thought, but you need more pornstars and fewer dead siblings/parents.
The part I felt humanized SCP-XXXX the most was the mention that he used his power to fulfill his basic sexual needs, which is perfectly normal. Many of us would try desperately to take care of that business while in containment, and humanoid scips feel all that more un-relatable when they don't. On the other hand, having a dark and troubled past where all of his family is dead makes him more like every other superhero than like any of us. Killing off all familial ties he might have on outside cuts off another contact point between him and the reader that you could explore.

In a similar vein, his weight gain also adds to his character (literally and figuratively). It seems like a mundane point, but it actually reminds the reader that it's a person – a human being who, like every other human being, can get fat – the Foundation has put in a cell. I advocated for the removal of this detail from the description, but I believe it belongs in this article. You've done well here.

As I read him, Damian Sabio solely serves the role of "the friendly guy MC kills because MC can't control their powers." Perhaps this is the only role he can serve here, but I'd at least try to flesh him out as more than that.

I think I like your characterization of Stephanie Rom the most. You've nailed the apathy she brings to the table with SCP-XXXX – the "I just work here" attitude, the type of attitude that gets threatened by normal people in anomalous circumstances. It's clear she's uncomfortable about getting close to him, and that discomfort betrays the sliver of sympathy she has for him.

On one final note: I'm really glad they didn't hook up. It's a natural consequence that they'd part; otherwise, it would be artificial in an un-pretty way. It's poignant allegory that no matter how normal the person is, their anomaly will make them inhuman.

TL;DR: Clean this up, polish the narrative and characters – and you will have yourself a story.